it's cheaper than buying an expensive Macro lens. It would have came about by someone just flipping their lens, hand holding it and finding that it was good at macro. So you could get some awesome macro for very cheap. You could try it right now with your 55mm lens, flip it, hand hold it to the camera and shoot.Why would you use this instead of your actual 40 mil macro lense though?
40mm macro lens $300
105mm macro lens $1100 (correction $860)
reverse ring - $30
extension tubes - $100 - $150?
and photo people try all kinds of weird things